Mechanism for electrically contacting a thin layer and use therefor

ABSTRACT

ELECTRICAL CONTACT WITH A THIN CONDUCTIVE LAYER IS OBTAINED USING A COMPLIANT SHEET OF INTERMEDIATE CONDUCTIVITY URGED BY A METALLIC CONDUCTIVE CLAMP INTO INTIMATE FACE-TO-FACE CONTACT WITH A SURFACE OF THE THIN CONDUCTIVE LAYER. THIS STRUCTURE IS ESPECIALLY USEFUL IN ELECTROPHOTOGRAPHIC PROCESSES IN WHICH THE THIN CONDUCTIVE LAYER IS TRANSPARENT AND SERVES AS ONE OF THE TWO ELECTRODES IN FIELD-INDUCED, IMAGEWISE CHARGE TRANSFER.

i 13, 1971 T. H. MORSE 3, MECHANISM FOR ELECTRICALLY CONTACTING A THIN LAYER AND USE THEREFOR Filed April 4. 1968 "l: r/L///// 1/////// 7 FIG.|

THEODORE H. MORSE INVENTOR.

ATTORNEYS United States Patent 3,574,615 MECHANISM FOR ELECTRICALLY CONTACTING A THIN LAYER AND USE THEREFOR Theodore H. Morse, Rochester, N.Y., assignor to Eastman Kodak Company, Rochester, NY. Filed Apr. 4, 1968, Ser. No. 718,802 Int. Cl. G03g 5/06 U.S. CI. 96-15 1 Claim ABSTRACT OF THE DISCLOSURE Electrical contact with a thin conductive layer is obtained using a compliant sheet of intermediate conductivity urged by a metallic conductive clamp into intimate face-to-face contact with a surface of the thin conductive layer. This structure is especially useful in electrophotographic processes in which the thin conductive layer is transparent and serves as one of the two electrodes in field-induced, imagewise charge transfer.

CROSS-REFERENCE TO RELATED APPLICATIONS Reference is made to commonly assigned copending U.S. patent applications Ser. No. 620,906, entitled Clamped Photoconductive Unit for Electrophotography, filed Mar. 6, 1967 in the name of H. T. Hodges, and Ser. No. 653,779, entitled Method of Repetitive Xerography Without Cleaning, filed July 17, 1967 in the names of J. G. Jarvis and W. C. York.

BACKGROUND OF THE INVENTION Field of the invention This invention relates both to the field of electrical connectingand to the field of electrophotography. More specifically, this invention relates to structure for providing reliable electrical continuity between a very thin conduotive layer and an electrical potential source, ground or the like, and to the use of such structure in electrophotographic processes in which the conductive layer is thin enough to be transparent and substantial current densities must be carried through the connection.

Description of the prior art A number of electrophotographic processes and apparatus have been suggested in which a photoconductive member is illuminated through a transparent conductive layer; see, for example, the aforementioned Jarvis and York application. In this process, a photoconductive layer having a transparent conductive backing is uniformly charged while the conductive layer is connected electrically to ground. To form an electrostatic image on the photo conductive layer, it is imagewise exposed through the transparent conductive layer. The electrostatic image is then utilized by methods well known in the are, for example, by toning, transferring and fixing.

Transparency in the most useful types of conductive layers is generally considered to be obtainable only with extreme thinness of such layers. In the case of a large number of commonly used metals, good transparency is obtained only at a thickness in the neighborhood of 0.02 micron. Certain metallic salts such as cuprous iodide are conductive enough for much electrophotographic Work and are transparent in somewhat greater thicknesses. Nevertheless, even with the best materials it is common for those practicing electrophotography with transparent conductive layers to be faced with the problem of electrically contacting a layer having a thickness of approximately 0.1 micron.

The above-referenced Hodges application is directed to 3,574,615 Patented Apr. 13, 1971 ice work of the type described above and in said Jarvis and York application. In such applications, a connection using the Hodges clamp is as long lived as most photoconductive layers.

In addition to ordinary xerography, the literature suggests another group of processes which use a transparent conductive layer. In these processes the transparent conductive layer allows exposure of a layer while the layer is subjected to an electric field. For example, as described in U.S. Pat. No. 2,825,814, issued to L. W. Walkup on Mar. 4, 1958, a photoconductive layer is placed in virtual contact with an insulating surface in the presence of an electric field. The photoconductive layer is imagewise exposed, making it imagewise conductive and increasing the potential drop between the photoconductive layer and the insulating surface, thereby causing imagewise charge transfer from the photoconductive material to the insulating surface producing a useful electrostatic image on the insulating surface. In order to expose a photoconductive layer in the presence of an electric field, the exposure is made through a transparent conductive electrode.

In using the last described electrophotographic process, connection of a transparent conductive layer to a potential source can be successfully made with the clamps described in the Hodges application. However, after a relatively small amount of use of such connections in this process, contact is lost, and new contact must be applied. To remedy this problem, conductive paste, as mentioned in the Hodges application, has been applied between the clamp and the exposed surface of the conductive layer. Again, after a small amount of use with a good silver conductive paste, contact is lost.

SUMMARY OF THE INVENTION According to the invention, the above-mentioned problem is solved by positioning in face-to-face contact with the conductive layer a thin sheet of compliant and, preferably, resilient material of intermediate conductivity (herein called semiconductivity). Means, such as a clamp, is provided to urge this material into intimate contact with an accessible portion of the conductive layer. This resilient material is then connected with the potential source, ground or the like. In a convenient embodiment, the metallic clamp both presses the semiconductive material into intimate contact with the conductive layer and provides the next link in the connection between the conductive layer and the voltage source.

Although there are a number of compliant semiconductive materials marketed for uses such as static-free packaging, corona shielding of high voltage splices and the like, which materials are useful in this application, a material found to be particularly useful in practice of this invention is an elastomeric conducting cable splicing tape sold as Irricon SB semiconducting cable splicing tape 42-031 by Insulating Materials Department of the Chemical and Metallurgical Division of General Electric Company.

The remarkable thing about this invention is that in contacting very thin layers subjected to relatively high pulse current densities, a semiconducting material gives excellent electrical contact over a long life, while high quality silver conducting paste in the same application gives only temporary contact.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 schematically illustrates a process in which the invention is especially useful; and

FIG. 2 is a schematic partial cross-section of an elec- 3 trical contacting structure according to the invention in an arrangement in which it is especially useful.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with general use in the art, the term conductive will be used to describe the thin layer of ordinarily conductive material, mentioned above, even though such layers are sometimes so thin that their surface resistivity is as high as ohms per square or higher. It should be noted that some materials can be coated to a thickness which passes one-half the radiation striking it but which has a surface resistivity as law as 10 ohms per square. For a complete discussion of conductivity, transparency and thickness of such layers, reference is made to US. Pat. No. 3,245,833, Trevoy.

Similarly, the term semiconductive will be used with regard to the intermediate connecting material to contrast its conductivity with that of conductive paste, a metal clamp and, in fact, with the material of the conductive layer when in bulk form. Although the term semiconductive, as used herein, defines materials having a surface resistivity between one ohm per square and 10 ohms per square, the invention is much more effective if the surface resistivity of this material lies between 10 ohms per square and 10 ohms per square and if such resistivity is matched up with that of the conductive layer as described more fully below. Thus, it is within the scope of the invention that the conductivity of the semiconductive contacting or connecting material can be greater than the conductivity of the conductive layer.

According to FIG. 1, a layered structure is arranged in the form of a sandwich. This sandwich includes a component having a transparent support 1, for example, photographic film base, a photoconductive layer 2 and a transparent conductive layer 3 therebetween. Photoconductive layers usable in this process can be made according to methods disclosed in US. Pat. No. 3,141,770, Davis et a1. Conductive layers can be made in thicknesses thin enough to be transparent according to methods described in US. Pat. No. 3,245,833, Trevoy, mentioned above. An insulating surface 8 is placed in virtual contact with a surface of the photoconductive layer 2. This insulating surface is commonly obtained by coating an insulating material such as polyethylene, or titanium dioxide in a suitable binder, onto ordinary paper 7. Against the back of the material 7 carrying the insulating surface is positioned a conductive member, for example, a plate 9". A potential source 10 is connected between the conductive layers 3 and 9 creating a field having a potential drop between the conductive layer 3 and the insulating surface 8. While this field is applied, the photoconductive layer 2 is exposed to an image of actinic radiation with a suitable means, for example, a microfilm projector 11. The exposed areas of the photoconductive layer become conductive, thereby increasing the potential drop across the minute air gap (exaggerated in FIGS. 1 and 2 for purposes of illustration) between the photoconductive layer and the insulating surface 8, which in turn causes charge transfer across the air gap, resulting in creation of an electrostatic image on the insulating surface which may be utilized, for example, by toning.

As mentioned above, repeated use of this process with prior clamping mechanisms, including the clamps disclosed in the Hodges application, even with the added use of a silver conductive paste, gives only temporary contact to the extremely thin conductive layer 3.

According to FIG. 2, a portion of the conductive layer 3 is accessible or exposed, that is, free of the photoconductive layer and any other covering layers. This may be accomplished in many Ways, for example, by dissolving a portion of the photoconductive layer in a solvent which does not affect the conductive layer or by using a more narrow hopper to apply the photoconductive layer than is used to apply the conductive layer during original manufacture. A small sheet 6 of compliant and preferably resilient material having intermediate conductivity is placed in face-to-face contact with the accessible portion of the conductive layer 3. Sheet 6 and the conductive layer 3 are urged into intimate electrical contact, for example, with clamp jaws 4 and 5. Jaw 5, which contacts the semiconductive sheet 6, is conductive and can easily serve as a terminal for connecting the conductive layer -3 with the source of potential 10. Sheet 6 then provides electrical continuity between an electrical element of in termediate conductivity 3 and an electrical element of greater conductivity 5 in an electrical circuit.

The following examples further illustrate the invention:

EXAMPLE 1 In a process conducted as described in regard to FIG. 1, a layered structure is used with a silver conductive paste between a clamp and conductive layer 3. After approximately cycles of use in the process, it is found that print quality suddenly deteriorates, as evidenced by severe image density reduction and increased background. An examination of the layered structure indicates that electrical contact with the conducting layer has been lost. A break can be detected in the conductive layer at its junction with the silver paste.

EXAMPLE 2 A transparent polyester support measuring 12 by 7 inches is coated with a layer of cuprous iodide in a binder to a thickness of approximately 0.1 micron. A photoconductive layer is coated on the cuprous iodide layer. Two strips of the photoconductive layer at opposite ends of the support are dissolved and washed away by a solvent that does not dissolve the cuprous iodide layer. A 1 by 7 inch strip of semiconductive tape 0.006 inch thick and having a surface resistivity of 10 ohms per square is placed in contact with each accessible portion of the surface of the cuprous iodide layer. A pair of Mr inch thick metal plates are used to clamp each strip of the tape to the conductive layer. Electrical contact is made with each pair of the metal plates.

The tape is formed of an ethylene-ethylacrylate copolymer (see, for example, U.S. Pat. No. 2,396,785, Hanford) with a carbon filler in a ratio of approximately 50% ethylene, 15% ethylacrylate and 35% carbon by weight. Also included is a plasticizer containing stearic and palmitic acids, aliphatic hydrocarbons and a small amount of an ester of phthalic acid.

To test the reliability of the contacts, current pulses are passed through the conductive layer by means of the two contacts. Electrical resistance of the assembly, as measured between the two contacts, is used to determine the contact condition. Current pulses, approximately those of the process described in regard to FIG. 1, of 0.6 milliamperes amplitude and one millisecond duration with a repetition rate of one pulse per second are applied over an eleven-day period, until 10 pulses have been applied. Resistance measurements indicate no measurable deterioration of the contacts during the first 8 by 10 pulses over a ten-day period. A resistance increase from 3.8 by .10 to 12 by 10 ohms occurs during the remainder of the test, the final value still representing a satisfactory resistance value.

EXAMPLE 3 In a process conducted as described in regard to FIG. 1 and Example 1, but with semiconductive tape of the type used in Example 2 substituted for silver paste, more than 3000 prints are made without apparent loss of contact or degradation of image quality.

The following is a theoretical explanation of the reasons applicant believes the material mentioned above works so well in contacting a thin layer, while highly conductive silver paste specifically designed for electrical contacting is strictly temporary in the same application. It is to be understood that the applicant does not inend to be limited by this expression of theory which is presented only for a fuller understanding of the invention. The applicant has found that the discharge rates generated in the conductive layer 3 are generally of much higher order when the layer is used as a field electrode in processes similar to those of FIG. 1 and Example 2 than they are when used in ordinary xerography. For example, current densities in the conductive layers in the process of FIG. 1 are commonly calculated to be as high as 10 amperes per square centimeter. Despite this, the extremely thin conductive layer is able to handle these currents within itself. However, when the conducting layer is placed into contact with an extremely conductive surface such as ordinary metal or silver paste, the charging currents are localized at the points or lines at which contact is made. The layer itself burns out at the contacting points and contact is totally lost. The conducting layer, because of its thinness, has an intermediate surface resistivity, for example, 10 ohms per square. When a compliant material of intermediate conductivity having a surface resistivity, in this case, also in the neighborhood of 10 ohms per square, is placed into intimate contact with the conductive layer, there is a better impedance match and charge transition is less abrupt and the conductive layer does not burn out. This feature of less abrupt charge transition is attributable to use of a semiconductive material which for best results in this application should be between 1-0 and 10 ohms per square, surface resistivity, and should effectively match with the conductivity of the thin conductive layer. For best results, the surface resistivity of the contacting material should not vary from that of the conductive layer by more than a factor of 10 ohms per square.

The compliant material itself has substantial thickness and durability, and is readily contacted by the metal of the clamp. An additional feature which the material has in its preferred form is resiliency, which gives it the ability to conform to the shape of the surface of the conductive layer even if that surface changesshape in use, thereby giving excellent long-lasting broad area contact.

Although this invention has been described with regard to use with a transparent conductive layer being used as a field electrode, in which use it has been found to give remarkable results compared with prior devices, applicant does not wish to limit himself to such a use, since it is clear that the invention will add substantial life to electrical contacts in other uses having similar characteristics, including the process described in the above-mentioned Jarvis and York application.

The invention has been described in detail with particular reefrence to a preferred embodiment thereof, but it will be understood that variations and modifications can be effected Within the spirit and scope of the invention as described hereinabove and as defined in the appended claim.

1 claim:

1. An electrophotographic member comprising:

a layered structure including at least:

(a) a transparent support layer,

(b) a photoconductive layer, and

(c) a transparent conductive layer positioned between said support layer and said photoconductive layer, an area of one surface of said conductive layer being accessible,

a sheet of elastomeric compliant conductive material positioned in face-to-face contact with said accessible area of said conductive layer, said sheet having a surface resistivity which is within 10 ohms per square of the surface resistivity of said conductive layer, and

clamp urging said sheet and said conductive layer into intimate electrical contact, said clamp having a conductive portion in electrical contact with said sheet.

References Cited UNITED STATES PATENTS 2,937,943 5/ 1960 Wal-kup 9 6-1 3,256,089 6/1966 Clark et al. 961 3,316,088 4/1967 Schalfent 96l.5 3,355,290 11/1967 Robillard 96l.5

GEORGE F. LESM'ES, Primary Examiner J. C. COOPER III, Assistant Examiner US. Cl. X.R. 174-68.5 

